An optical isolator is an element which allows light to transmit only in one direction but prevents the light from propagating in a opposite direction thereto. For example, by arranging the optical isolator at an emitting end of a semiconductor laser, the light emitted from the semiconductor laser transmits through the optical isolator, and it is possible to be used this light as a light source for optical fiber communications. Conversely, the light which is going to be incident on the semiconductor laser through the optical isolator prevents by the optical isolator, so that the light cannot be incident on the semiconductor laser. Unless the optical isolator is placed at the emitting end of the semiconductor laser, a reflected return light will be incident on the semiconductor laser, and thereby a degradation of oscillation characteristics of the semiconductor laser is caused. Namely, the optical isolator serves to block the light which is going to be incident on the semiconductor laser, and to maintain a stable oscillation without degrading the characteristics of the semiconductor laser.
In optical active elements such as not only the aforementioned semiconductor laser but also an optical amplifier or the like, operating characteristics of the element are degraded by unintentional incidence of light from an opposite direction. Since the optical isolator allows the light to transmit only in one direction, it is possible to prevent the unintentional incidence of light to the optical active element from the opposite direction.
In an area which the light transmits, a structure having no wave-guiding action in which the light is confined within a cross sectional area perpendicular to the propagation direction of the light (the bulk type) and a structure having a wave-guiding action (the waveguide type) are exist in the optical isolator. In recent years, there has been a pressing need to downsize both the optical isolator of the bulk type and the optical isolator of the waveguide type, and thereby it is also need to downsize each component.
For example, as shown in Japanese Published Patent Application No. 2002-277826, and as shown in FIG. 1 with the schematic cross-sectional view, a waveguide type optical isolator 100 is constituted by providing a substrate 101, a waveguiding layer 103 formed with a waveguide 102, a magnetic garnet 104, and magnetic field applying means having a permanent magnet 105. And as shown in the figure, the waveguide type optical isolator 100 has waveguides 102A and 102B, and permanent magnets 105A and 105B corresponding to the waveguides 102A and 102B. A magnetic field is provided to the light passing through the waveguides by the permanent magnets 105A and 105B. In order to provide an appropriate phase variation to the light, it is required to populate the permanent magnets 105A and 105B at appropriate positions on the magnetic garnet 104 so that the mounted distance between the permanent magnets 105A and 105B is made an appropriate distance.
However, as the waveguide type optical isolator 100 is downsized, the permanent magnet 105 is also downsized. Therefore, it is very difficult to populate the permanent magnet 105 at an appropriate position on the magnetic garnet 104, and thereby population accuracy of the magnet is extremely low.
Moreover, in the waveguide type optical isolator 100, the permanent magnet 105 is fixed on the magnetic garnet 104 by adhesive. Therefore, since it becomes a state which a mutual stress of the permanent magnet 105 act at all times, for example, the problem occurs that the permanent magnet 105 separate from on the magnetic garnet 104, consequently, the structure of the waveguide type optical isolator remains uncertainty with respect to fix the permanent magnet 105 over a long period of time.
FIG. 2 shows an operating principle of the waveguiding type optical isolator. The waveguiding type optical isolator is set so that light waves which propagate in optical waveguides 102A and 102B may be in phase to propagating waves of forward direction (forward propagating waves) and may be in opposite phases to propagating waves of backward direction which propagate in opposite directions (backward propagating waves), by utilizing a phase variation whose magnitude is different depending on a propagation direction generated in optical waveguides 102A and 102B constituting an optical interferometer (hereinafter, referred to as “non-reciprocal phase shifting effect”) (FIG. 2(a)).
In a case where two light waves are in phase, the light waves which launched from a central input end 107 is output from a central output end 109 in a branch coupler 108 (in the example of the figure, it is used a tapered branch coupler) provided on the output side (on the right-hand side in the figure) based on the symmetry of the structure (FIG. 2(b)).
Meanwhile, in a case where two light waves are in opposite phases, from the symmetry of the structure, since an antisymmetric distribution is formed in a branch coupler 108 provided on the input side (in the left-hand side in the figure), the light waves launched from central output end 109 are not output from the central output end 107 but are output from waveguides 102A and 102B which is undesired light output ends provided on both sides of the central output end 107 (FIG. 2 (c)).
Namely, the light wave launched from the central input end 107 of the branch coupler 108 on the left-hand side in the figure is output from the central output end 109 of the branch coupler 108 on the right-hand side in the figure. Conversely, the light wave launched from the central output end 109 of the branch coupler 108 on the right-hand side in the figure is output from the waveguides 102A and 102B on the right-hand side in the figure without entering to the central input end 107 of the branch coupler on the left-hand side in the figure. As described above, by utilizing the phase difference between ordinary light and extraordinary light, it is possible to isolate a propagation light of the opposite direction launched from the central output end 109 of the branch coupler 108 on the right-hand side in the figure.
In order to achieve an operation of a branching and coupling characteristic of such a light of the optical isolator, a constant amount of non-reciprocal phase shifting effect is required. First of all, one of the interfering optical paths is made longer than the other one, and thereby a phase difference independent of the propagating direction between the two optical paths (reciprocal phase difference) is generated. And the non-reciprocal phase shifting effect can be generated by arranging a magneto-optical material (material which has a magneto-optical effect) in a planar waveguide, externally applying a magnetic field in a direction perpendicular to a propagation direction (transverse direction) in a waveguide plane, and orienting magnetization of the magneto-optical material. Since the non-reciprocal phase shifting effect due to magneto-optical effect is determined by the relation between the propagation direction of the light and an orientation direction of the magnetization, the non-reciprocal phase shifting effect becomes different in a case where the propagation direction is reversed while keeping a magnetizing direction.
Since the magnetic fields are applied in an antiparallel to each other to waveguides 102A and 102B constituting the interferometer in the waveguide type optical isolator shown in FIG. 2, a phase difference of the light waves when each light waves propagate the same distance in waveguides 102A and 102B corresponds to an amount of non-reciprocal phase shifting (difference of the phase variation between the forward propagating wave and the backward propagating wave). Additionally, if a phase difference of +φ occurs between two waveguides due to the non-reciprocal phase shifting effect to the forward propagating wave, a phase difference of −φ which is an opposite sign to that will occur to the backward propagating wave.
Here, an optical path length difference corresponding to ¼ wavelength is provided in two waveguides constituting the interferometer. It is intended that the light which propagates through a waveguide with longer optical path gives a phase variation which is larger only π/2 regardless of the direction. Namely, if the waveguide with longer optical path is made to generate a phase difference due to the non-reciprocal phase shifting effect (hereinafter, referred to as “non-reciprocal phase shifting phase difference”) of −π/2 as compared with a waveguide with shorter optical path to the forward propagating wave, the light waves which propagate through two waveguides are in phase to the forward propagating wave. At this time, since the sign of the non-reciprocal phase shifting phase difference is reversed when the propagation direction is reversed, the non-reciprocal phase shifting phase difference of +π/2 is given to the waveguide with longer optical path. This phase difference and the phase difference of +π/2 due to the optical path length difference are added, so that the light will input into the branch coupler in the state of an opposite phase (phase difference π). From the discussion described above, it can be concluded that the non-reciprocal phase shifting phase difference of π/2 is required in the waveguide type optical isolator shown in FIG. 2.